Discussion of the Background
The need exists for a highly reliable and accurate system which can locate a radio transmitter in an unknown position in a citywide or local area of coverage. Applications include personal safety from assault or medical causes, roadside assistance, child monitoring for kidnapping recovery, monitoring of the elderly to reduce walk aways, drug enforcement, early release or parolee monitoring, as well as stolen vehicle or stolen equipment recovery. The unknown position radio transmitter broadcasts a signal that reflects off of objects, such as buildings or buses, before arriving at a series of receivers. These reflections cause several versions of the same signal, delayed by different amounts depending on the number of reflections incurred, to be superimposed with one another. This distorts the transmitted signal and, if uncorrected, prevents reception and processing of the signal.
Present systems include inherent drawbacks, such as continual monitoring, which require full time surveillance operation or GPS services which require physically larger units with short battery lives and which must operate outdoors in reasonably good view of several satellites. Stanford Telecom, in "RF Design", October 1992, teaches the use of signal averaging to reduce multi-path errors. "The tracking error is not always the same sign since the multi-path will either subtract or add to the signal depending on its carrier phase with respect to that of the direct signal. In the moving vehicles, multi-path tends to change sign rather rapidly and is noiselike, averaging out in the long term."
In Spread Spectrum Systems, Third Edition, Dixon teaches the use of spread spectrum in time of flight to yield position fix information. Sanderford, et al., in U.S. Pat. No. 4,799,062 disclose the use of additional receiving sites so that signals from one or more receiving sites that suffer from delays, due to multi-path errors, can be removed as inputs to a least squares fit algorithm. Sanderford further teaches the use of a mobile reference transmitter carried by a search team which is guided by a central dispatch station. The guidance is provided through a differential term derived from a poll reply of the unknown positioned transmitter. However, this technique requires that the unknown positioned transmitter have two-way communication capabilities.
Global positioning systems also provide radio location capability if connected in a manner to re-transmit latitude, longitude, altitude information to a remote central monitoring station. GPS has several shortcomings as compared to the instant invention for the applications contemplated herein. These include direct signal propagation through the interior of multi-story buildings, the increased size and weight due to the requirement of the inclusion of a receiver as well as a transmitter, and reduced battery life due to the current drain of the element of the GPS receiver. The applications require physically small devices, long battery life, and imply transmit-only operation. Successful operation is mandatory, even in a heavily radio frequency shielded environment such as a multi-story high-rise building. In order to increase the Signal-to-Noise Ratio available to a system receiver from a system transmitter whose operation is constrained to the preceding conditions, it is desirable to locate the receiver on a ground-based platform as opposed to an orbital one. The ground-based receiver platform does have a disadvantage due to a fairly complex radio wave travel path from the system transmitter (multi-path signal distortion). Urban and suburban multi-path distortion may be quite severe, as it's cause increases dramatically with each new object inserted into the path from system transmitter to system receiver (i.e. buildings, vehicles, structures). These objects cause destructive "copies" (echoes) of the radio signal to be generated each time the signal "reflects" (bounces) off of one. The echoes arrive at the system receiver later in time than the desired (direct path) signal, causing loss of signal, or producing erroneous measurements. The analysis of how a particular environment will induce multi-path distortion is called a "Delay Spread Profile". Delay spread profile analysis shows multi-path echoes on the order of one to five microseconds are not uncommon in urban and suburban environments. Delay spreads of this magnitude cause one thousand feet to one mile potential error to a transmitter's calculated position if error removal tactics are not employed.